Whether you call it sex, intercourse or making love, we all know what happens when we slip between the sheets.

But how does the act of coitus change our brains and bodies? And why does it hurt so much when we are rejected after a fling?

The act of sex sets off a cascade of hormones, which trigger pleasure, happiness, and bonding, scientists reveal.

A plethora of studies now shows intercourse is good for our health, from protecting against heart disease and osteoporosis, to staving off dementia and improving mood.

Below, they describe exactly how sex can help us live longer, and why sexual rejection breaks our hearts..

WHY LOVE HURTS

A key hormone released during sex is oxytocin, also known as the ‘cuddle hormone’. This lowers our defences and makes us trust people more, says Dr Arun Ghosh, a GP specialising in sexual health at the Spire Liverpool Hospital.

It’s also the key to bonding, as it increases levels of empathy. Women produce more of this hormone, although it’s not clear why, and this means they are more likely to let their guard down and fall in love with a man after sex.

However, the problem is that the body can’t distinguish whether the person we’re with is a casual fling or marriage material — oxytocin is released either way. So while it might help you bond with the love of your life, it’s also the reason you may feel so miserable when a short-term relationship ends.

Men, on the other hand, instead of getting a surge of bonding hormone receive a surge of simple pleasure.

‘The problem is that when a man has an orgasm, the main hormone released is dopamine — the pleasure hormone. And this surge can be addictive,’ says Dr Ghosh.

That’s why so many more men tend to suffer from sex addiction.

Oxytocin is released during orgasm which makes us trust people – but it also means we feel miserable when a short-term relationship ends

Kissing raises levels of pleasure hormones and so can improve sex later

Sex can protect against heart disease, dementia and osteoporosis

It can even improve vision by relaxing the eye, neck and shoulder muscles

STAVING OFF DEMENTIA

Many of us are all too aware that our brain cells decline with age. In fact, it’s said we lose 7,000 brain cells a day by the time we’re 35, which makes rather depressing reading.

The good news, however, is that having regular sex may help us grow new brain cells, according to scientists from Princeton University in the United States.

And the more sex you have, the more cells you can grow, it seems. Animal studies, published in the journal PLoS ONE, suggest that sex stimulates the growth of brain cells in the hippocampus, the part of the brain responsible for memory and learning.

Factors such as stress and depression have been shown to shrink the hippocampus, yet exercise and sex counteract this effect.

‘MRI scans have shown that during orgasm the neurons in the brain are more active and use more oxygen,’ explains Barry Komisaruk, professor of psychology at Rutgers University and a leading authority on sex and neuroscience.

‘It appears that the more active the neurons, the more oxygen they withdraw from the blood — so more oxygenated blood is supplied to the region, delivering a fresh supply of nutrients.’

But as well as boosting brain cells, sex could also sharpen a woman’s mind, says Dr Ghosh. That’s due to a surge in sex hormones, particularly testosterone, which can help improve concentration and reaction times.

SMOOCHING MEANS BETTER SEX

The lips are packed with nerve endings — 100 times more than the fingertips.

Research using laboratory animals and humans has found that orgasm can inhibit the release of pain transmitters from the spinal cord so they can’t reach the neurons in the brain that respond to pain.

In fact, he says, it can raise the pain threshold so much that the equivalent effect in morphine is three times the usual pain-relieving dose.

MIND POWER

A lot of sexual health research focuses on what’s happening physically.

But experts say that for many people — especially women — the mind plays a key role in achieving orgasm.

While male brains tend to focus on the physical stimulation involved in sexual contact, the key to female arousal seems to be deep relaxation and a lack of anxiety.

The scans show that, during sex, the parts of the female brain responsible for processing fear, anxiety and emotion start to relax more and more, reaching a peak at orgasm, when the female brain’s anxiety and emotion are effectively closed down.

THE ULTIMATE HAPPY PILL

‘There is a known circuit in the brain that’s involved in feeling happy,’ explains Paul Thompson, professor of neurology at the University of California, Los Angeles, and an expert in brain imaging.

‘This is called the limbic region, which is deep in the brain, and it’s more active when we’re rewarded. The same system is activated by sex, drugs, gambling — basically anything we enjoy.

‘There’s more blood flow to the head, more neurons firing, more brain activity.

‘Much of this deep brain circuit runs on the hormone dopamine. Anything pleasurable tends to cause these brain cells to exude more of it and this makes them fire faster.’

‘Without doubt, sex has not only physical but psychological and social benefits, too,’ says Dr Ghosh.

The endorphins released during sex can really help treat depression and clear the mind.

‘One is serotonin, also called the happy hormone, which creates a sense of bliss. People often say that sex is the last thing they feel like doing when they’re depressed, but do try it if you can.

‘And don’t worry if you get emotional afterwards — it’s very common to end up crying after sex. This is caused by a combination of endorphins being released and heightened emotions.’

BETTER SLEEP

If you’re struggling to nod off, it’s better to have sex than take a sleeping pill, says Dr Ghosh.

‘In fact, it’s more beneficial to have sex in the evening rather than the morning because the body wants to be relaxed afterwards, not get up and go to work.

‘By having sex at the end of the day you’ll reap more of the stress-relieving benefits. For a man, a powerful orgasm is the equivalent of having on average a 2-3mg shot of diazepam (or Valium).

‘That’s why so many men nod off afterwards — there really is a biological reason for this.’

IT’S A GOOD WAY TO TEST YOUR EYES

Sex relaxes the muscles and alleviates neck and shoulder tension, says Dr Arun Ghosh, a GP specialising in sexual health at the Spire Liverpool Hospital. And surprisingly, it might also tell you whether you need glasses.

‘I’ve had patients complain of poor vision after sex. What’s happened is that, like all the other muscles in the body, their eye muscles have relaxed and are performing at their true ability, rather than straining and squinting as they would normally.’

So if your sight goes blurry after sex, it’s worth going for an eye test.

IT PROTECTS AGAINST HEART DISEASE

Sex can have a protective effect on the heart. A study at Queen’s University in Belfast found that having sex three times a week could halve the risk of heart attack or stroke.

Another study in Israel found that women who had two orgasms a week were up to 30 per cent less likely to have heart disease than those who didn’t enjoy sex or didn’t orgasm.

Dr Lisa Turner, a sex and relationship therapist, says: ‘One theory is that these women may have felt depressed, which has been linked with an increased risk of a heart attack.

‘The endorphins released during sex also neutralise the stress hormones in the body, which are linked to heart disease.’

And the old cliche of sex causing heart attacks?

‘It’s a very low risk — it accounts for less than 1 per cent of all deaths,’ says Dr Graham Jackson, consultant cardiologist at the London Bridge Hospital and chairman of the Sexual Advice Association.

However, if you’re over 50, overweight and unfit, there is a risk to your heart — just as there would be if you suddenly and vigorously took up any form of exercise.

‘What also increases the risk is extra-marital affairs, and this has been proven by three large studies,’ says Dr Jackson.

‘In fact, this accounts for 75 per cent of heart-related deaths from sex, 90 per cent of which are older men. It’s thought the combination of high-fat foods and alcohol — wining and dining — combined with vigorous exercise, often with a younger partner, is what tends to trigger the heart event.’

IT’LL STRENGTHEN YOUR BONES

U.S. research found menopausal women who had sex every week had oestrogen levels twice as high as their abstaining counterparts.

The hormone has a protective effect in bone health and a lack of it after the menopause has been linked with osteoporosis.

‘Regular sex increases the production of oestrogen,’ says Dr Peter Bowen-Simpkins of the Royal College of Obstetricians and Gynaecologists.

Sex increases the production of oestrogen, which can protect against brittle bones

IT MAKES YOU MORE FERTILE

But the amount of sex you have is important. The more often you make love, the better quality your sperm will be.

‘If you’re trying to conceive, you need the sperm to be as fresh as possible — not stuff that’s been sitting in the prostate for three or four days,’ he adds.

IT STAVES OFF PROSTATE CANCER

Researchers at Nottingham University have found that men who enjoy a regular sex life in their 50s are at lower risk of developing prostate cancer. ‘Clearing the prostate out regularly is the reason behind this,’ says Dr Ghosh.

‘The link was suggested after research showed that monks appear to have a higher chance of developing prostate cancer.’

IT’LL HELP YOU QUIT SMOKING

Here’s a simple way to get your man to kick his smoking habit — remind him that it causes the penis to shrink and can cause impotence.

Erections are all about good blood flow, and lighting up worsens blood flow to the spongy tissues in the penis, says Raj Persad, consultant urological surgeon at Bristol Royal Infirmary.

‘As a result, they become starved of oxygen and the delicate cells die away. They then become fibrotic — forming scar tissue — which is less elastic and less able to expand during an erection than regular healthy tissue.’

Smoking is also a known cause of erectile dysfunction as it causes hardening of the arteries and hampers good blood flow.

A study of more 7,000 Chinese men, published in the American Journal of Epidemiology, found that the more cigarettes smoked a day, the higher the risk of impotence.

IT GUARDS AGAINST INCONTINENCE

Providing that a woman is fairly active during sex, then it’s a good workout for her pelvic floor muscles, says Dr Gillian Vanhegan, a spokesman for the Royal College of Obstetricians and Gynaecologists.

The muscles that stem the flow of urine, reducing leakage and incontinence, all benefit and this reduces the risk of stress incontinence and prolapse later in life, both of which are very common.

‘After having children many women don’t have the time to do dedicated pelvic floor exercises, so sex provides a natural workout.’

Researchers at Nottingham University have found that men who enjoy a regular sex life in their 50s are at lower risk of developing prostate cancer (pictured under the microscope)

AND FINALLY…USE IT OR LOSE IT

‘Just like smoking or diabetes, a lack of regular blood flow to the penis will over time cause some of the tissues to die,’ says Mr Persad.

Indeed, Finnish research has confirmed that men who had sex less than once a week at the start of the study were twice as likely to develop erectile dysfunction as those who had it at least once a week.

In the study of 1,000 men aged between 55 and 75, those who had sex three or more times a week lowered their risk fourfold.

There’s no reason not to enjoy a full sex life in old age, adds Dr Jackson.

‘We have some patients in their 80s taking Viagra-type drugs and studies have shown that a regular sex life can prolong life — it’s basically another form of exercise.

‘There seems to be a myth that you’re past it once you hit 65, but that’s rubbish.’